Forced flow cryogenic cooling in fusion devices: A review

The constantly increasing energy consumption along with the depleting fossil fuel resources as well as owing to the fact that the nuclear fission not being an intrinsically safe method of energy generation, it has become necessary to look for other solutions to fulfil the future energy demands. Nuclear fusion, the source of energy for billions of stars, has attracted the attention of scientists and engineers despite a lot of technical challenges in the replication of the fusion process in laboratories. For fusion to take place in a device, one of the major challenges faced is the strong magnetic confinement of the plasma using large superconducting (SC) magnets, which need efficient cryogenic cooling techniques to maintain the required low temperatures for the superconducting state. In order to maintain the compactness, the SC magnets generally employ Cable in Conduit Conductor (CICC) windings, carrying high current densities, which are cooled by the forced flow of helium at ~4 K temperature to maintain the required superconducting temperatures. The construction of CICC aims to maintain the superconductivity state by optimization of various parameters such as thermal stability, the ratio of normal conductor to SC material, mechanical strength, low hydraulic impedance, current density, magnetic field, etc. The cryogenic thermal stability of the CICC is of prime importance for safe, stable and reliable operation of SC magnets. The prediction of thermal and hydraulic behavior of the CICC in large SC magnets is difficult due to the complex geometry involved, the variation in fluid properties, various heat in-flux incidences over the long length of CICC and a complex heat transport phenomenon. Another application which utilizes a forced flow cryogenic cooling in the fusion devices is a cryo-adsorption pump for creating clean and high vacuum with large pumping speed. This paper presents an overview of the forced flow cryogenic cooling schemes in fusion devices along with a systematic review of the thermal and hydraulic studies related to CICC and cryo-adsorption pump, thereby highlighting the challenges and opportunities for further improvement in their design and performance.